it really is big news. it's very exciting. >> reporter: dr. grant colfax is director of hiv prevention and research with the san francisco department of public health. he says his finding show antiretroviral drugs can not only treat hiv; they may prevent infection, as well >> that sounds dramatic. >> it's a paradigm shift. >> reporter: the two new studies followed thousands of heterosexual couples in africa where only one partner was infected. the uninfected partner took either truvada or a placebo. the results were so dramatic, researchers stopped the study and offered medication to all uninfected partners. it's a new tool in the toolbox. >> we know that the more options we have for preventing hiv the better. >> reporter: in terms of prevention, truvada offers a double whammy. if you're infected, you're less infectious. and if you're not infected, you're also likely to get it. even so, dr. colfax warns it's not a magic bullet. >> people need to continue to use condoms. they need to continue to get hiv tested, use clean needles if he they inje